1. Field of the Invention
This invention pertains to tests of granular materials. More particularly, apparatus and method are provided for testing crush-resistance of granular materials such as proppants used in wells.
2. Background of Inventions
Slurries of granular material are commonly pumped down oil and gas wells to improve the producing characteristics of the well. The most common procedure is to fracture the earth by pumping fluids at a high rate and then to pump a slurry of granular material down the well and into the open fracture. This process, called “hydraulic fracturing,” may increase the production rate of wells up to several-fold. In another method to improve producing characteristics of a well, granular materials are packed around or in a wellbore to serve as a filter, in a process called “gravel packing.” In both processes, the fluid in the slurry is separated from the granular material and the granular material is left in the well in the form of a packed bed.
In the hydraulic fracturing process, since the granular material is in a fracture in the earth, it is subjected to earth stresses, which may reach 10,000 psi or more. Stress tends to cause crushing of the material. The granular material used in the hydraulic fracturing process is called a “proppant” because it “props” the fracture to keep it open. Since the capacity for fluid flow through the proppant material is important and maximum flow capacity is needed to obtain maximum production rate from a well, it is important that crushing of proppants be measured.
Industry has long recognized the need for proppants that crush a minimum amount under stress. Silica sand was used almost exclusively as a proppant for many years and it was found to vary widely in composition and strength. A need arose for a test to compare the amount of crushing of different sands used as a proppant. In 1983, a committee organized by the American Petroleum Institute (API) published “Recommended Practices for Testing Sand Used in Hydraulic Fracturing Operations” (API Recommended Practice 56, March 1983). Section 8 of the document describes a “Recommended Frac Sand Crush Resistance Test.” This test employs the cylindrical test cell shown in FIG. 1. The procedure includes sieving a sample of sand to the desired range of sizes and placing a measured amount of sand inside the cylinder to produce a sand concentration of 4 lbs per sq. ft. The API procedure states: “Pour the sand sample into the test cell constantly moving the source of the sand to keep the surface in the cell as level as possible.” Then the procedure prescribes leveling the surface of the sand in the cell by inserting the piston in the cell and rotating the piston 180 degrees in one direction only without applying any force to the piston.
After the API cell is loaded with proppant, it is placed in a press and the force required to obtain a desired stress level in the cell is applied. The sample is then removed from the cell and sieved and the weight of crushed material passing through the smaller screen is measured and reported as a percentage of the weight of the original sample. The API committee recommended the maximum amount of fines for each mesh size range of sand that would be acceptable at stress levels such as 4,000 psi or 5,000 psi. For example, 20-40 mesh sand stressed to 4,000 psi was suggested to have maximum fines produced of 14%. Some natural sands that failed to pass the API crush test may have been rejected for use in hydraulic fracturing. This is illustrated for example in the paper SPE 98019, “Analysis of Non-API Industrial Sands for Use in Hydraulic Fracturing.” It was reported that two samples of sand “failed the crush test which allows a maximum of 14% fines.”
With the further development of synthetic or manmade proppants, the procedures developed for sand were also applied to compare these proppants. The technical groups comparing different natural and synthetic proppants have been limited by variability of results among different laboratories and by reproducibility of results within the same testing facility, particularly when tests are performed by different operators.
There is currently circulating an update of the API RP 56 reference in the form of document ISO TC 67/SC 3, dated Dec. 5, 2005. This document is not an international standard at this time; recipients of the document are invited to submit their comments. The proposed document contains the test and procedure utilizing the same cell as shown in FIG. 1 and a similar procedure for placing proppant in the cell. The amount of proppant to be used in the test is calculated from the “loose-pack” bulk density. “Crush stress level guidelines” are provided for manmade proppants and sand proppants, but no maximum amount of crushing is prescribed. The remark is added that: “ . . . variance in crush results have been largely associated with the method of loading the crush cell.”
U.S. Pat. No. 6,109,486 discloses a dry sand pluviation device. (The patent explains that the term “pluviation” is a term related to the Latin word for “rain” and refers to the fact that the granular particles fall like raindrops.) The pluviation device of the '486 patent is used to load test apparatus for soil mechanics studies. In the study of soil mechanics it is also important that the soil particles be placed in a uniform fashion that allows a precisely controlled and consistent soil layer density. The vessel of the invention is an open top box having vertical side walls and a horizontal bottom tray with multiple perforations uniformly spaced on a square or equilateral triangle pattern. The vessel has a moveable tray disposed below the stationary bottom of the vessel that has corresponding multiple perforations such that the slideable tray can be used to close the perforations in the bottom of the box.
U.S. Pat. No. 4,768,567 discloses a sand-filled apparatus for casting. Sand particles are placed within a foundry mold in preparation for casting by the lost foam process. A compressed air conduit is temporarily inserted to direct air flow toward the pattern and divert pluviating sand to promote even packing of the sand about the pattern.
In geotechnical testing, pluviation has been studied as a method to prepare reconstituted samples for testing. The effects of the structure of the “sand rainer” (the apparatus used to pluviate the sand into the testing apparatus) have been reported in “Factors Affecting Sand Specimen Preparation by Raining,” Geotechnical Testing J., Vol. 10, No. 1, Mar. 1987, pp. 31-37.
Soil mechanics properties such as the cyclic loading response of sand have been observed to be dependant on the method of formation of the specimens for testing. (“Cyclic loading response of loose air-pluviated Fraser River sand for validation of numerical models simulating centrifuge tests,” Can. Geotech. J., 42 550-561 (2005) Air-pluviated specimens were more susceptible to liquefaction under cyclic loading than water-pluviated samples. Differences between the two specimens were attributed to differences in particle structure; the differences highlighted the importance of “fabric effects” in the assessment of mechanical response of sands. A simple “raining technique” that allowed relatively independent control of both fall height and mass flow rate of sand was found to be preferable for the preparation of specimens. It was found that the as-placed density of the river sand increased with increasing fall height and decreasing mass flow rate. Sand was rained through a 1 millimeter sieve or a 2.5 millimeter sieve. The effects of flow rate and average fall height on the relative density of packs of the river sand was provided.
Alternative granular materials available in industry for use in hydraulic fracturing now include silicon sand, resin coated sand and a variety of ceramic granular materials, which may also be resin-coated. The crush test originally proposed in the API RP 56 has been used many thousands of times to compare the strength of these various granular materials. It has been found that crush-resistance tests by different laboratories (sometimes called “round-robin tests”) vary over a broader range than is desirable to make reliable comparisons of different materials. Crush-resistance tests are also used for quality control during manufacture of manmade proppants, where variations in test results may cause difficulties in manufacturing process optimization. It is believed that the principal cause of the variations in crush results is the difference in loading procedure between different operators and different laboratories. The present procedures require that a part of the procedure that can have a significant effect on crush results be carried out by a person. Therefore, what is needed is a crush-resistance test for granular materials to compare the crush-resistance of various materials that produces results independent of the operator of the tests.